Chapter III: Miscellaneous Processes (4)

The bibliography referring to special points will be given later. The simplest way of exhibiting the relative resistances of metals is by means of a diagram published by Dewar and Fleming (loc. cit.), which is reproduced on a suitable scale on the opposite page. For very accurate work, in which corrections for the volumes occupied by the metals at different temperatures are of importance, the reader is referred to the discussion in the original paper, which will be found most pleasant reading. From this diagram both the specific resistance and the temperature coefficient may be deduced with sufficient accuracy for workshop purposes. In interpreting the diagram the following notes will be of assistance. The diagram is drawn to a scale of so-called "platinum temperatures"--that is to say, let R0, R100, Rt be the resistances of pure platinum at 0°, 100°, and t° C. respectively, then the platinum temperature pt is defined as

pt = 100 X (Rt-R0)/(R100-R0)

This amounts to making the temperature scale such that the temperature at any point is proportional to the resistance of platinum at that point. Consequently on a resistance temperature diagram the straight line showing the relation between platinum resistance and platinum temperature will "run out" at the platinum absolute zero, which coincides more or less with the thermodynamic absolute zero, and also with the "perfect gas" absolute zero. Platinum temperatures may be taken for workshop purposes over ordinary ranges as almost coinciding with air thermometer temperatures. The metals used by Professors Dewar and Fleming were, with some exceptions, not absolutely pure, but in general represent the best that can be got by the most refined process of practical metallurgy. We may note further that the specific resistance is only correct for a temperature of about 15° C, since no correction for the expansion or contraction of material has been applied.

The following notes on alloys suitable for resistance coils will probably be found sufficient.

§ 117. Platinoid.

This substance, discovered by Martino and described by Bottomley (Phil. Proc. Roy. Soc. 1885), is an alloy of nickel, zinc, copper, and 1 per cent to 2 per cent of tungsten, but I have not been able to obtain an analysis of its exact composition. It appears to be difficult to get the tungsten to alloy, and it has to be added to part of the copper as phosphide of tungsten, in considerably greater quantity than is finally required. The nickel is added to part of the copper and the phosphide of tungsten, then the zinc, and then the rest of the copper. The alloy requires to be remelted several times, and a good deal of tungsten is lost by oxidation.

The alloy is of a fine white colour, and is very little affected by air--in fact, it is to some extent untarnishable. The specific resistance will be seen to be about one and a half times greater than that of German silver, and the temperature coefficient is about 0.021 per cent per degree C. (i.e. about nineteen times less than copper, and half that of German silver). To all intents and purposes it may be regarded as German silver with 1 per cent to 2 per cent of tungsten. It does not appear to have been particularly examined for secular changes of resistance.

118. German Silver. This material has been exhaustively examined of late years by Klemencic and by Feussner and St. Lindeck. Everybody agrees that German silver, as ordinarily used for resistances, and composed of copper four parts, zinc two parts, nickel one part, is very ill-fitted for the purpose of making resistance standards. This is due (1) to its experiencing a considerable increase in resistance on winding. Feussner and St. Lindeck found an increase of 1 per cent when German silver was wound on a core of ten wire diameters.

(2) To the fact that the change goes on, though with gradually decreasing rate, for months or years;

(3) to the fact that the resistance is permanently changed (increased) by heating to 40° C. or over. By "artificially ageing" coils of German silver by heating to 150° C, say for five or six hours, its permanency is greatly improved, and it becomes fit for ordinary resistance coils where changes of, say, 1/5000 do not matter.

It is a remarkable property of all nickel alloys containing zinc that their specific resistance is permanently increased by heating, whereas alloys which do not contain zinc suffer a change in the opposite direction. The manufacturers of German silver appear to take very little care as to the uniformity of the product put on the market; some so-called German silver is distinctly yellow, while other samples are bright and white.

It is noted by Price (Measurements of Electrical Resistance, p. 24) that German silver wire is apt to exhibit great differences of resistance within quite short lengths. This has been my own experience as well, and is a great drawback to the use of German silver in the laboratory, for it makes it useless to measure off definite lengths of wire with a view to obtaining an approximate resistance. In England German silver coils are generally soaked in melted hard paraffin. In Germany, at all events at the Charlottenburg Institute, according to St. Lindeck--coils are shellac-varnished and baked. In any case it appears to be essential to thoroughly protect the metal against atmospheric influence.

§ 119. Platinum Silver.

In the opinion of Matthieson and of Klemencic the 10 per cent silver, 90 per cent platinum alloy is the one most suitable for resistance standards. At all events, it has stood the test of time, for, with the following exceptions, all the British Association coils constructed of it from 1867 to the present day have continued to agree well together. The exceptions were three one-ohm coils, which permanently increased between 1888 and 1890, probably through some straining when immersed in ice. One coil changed by 0.0006 in 1 between the years 1867 and 1891. According to Klemencic, absolute permanency is not to be expected even from this alloy.

Its recommendation as a standard depends on its chemical inertness, its small temperature coefficient (0.00027 per degree), and its small thermo-voltage against copper, as the following table (taken from Klemencic) will show:-

Thermo-voltages in Micro-volts per degree against Copper
over the Range 0° to 17° C.

Platinum iridium 7.14 micro-volts per degree C.

Platinum silver 6.62 micro-volts per degree C.

Nickelin 28.5 micro-volts per degree C.

German silver 10.43 micro-volts per degree C.

Manganin (St. Lindeck) 1.5 micro-volts per degree C.

Mechanically, the platinum silver is weak, and is greatly affected as to its resistance by mechanical strains--in fact, Klemencic considers it the worst substance he examined from this point of view--a conclusion rather borne out by Mr. Glazebrook's experience with the British Association standards already referred to (B. A. Reports, 1891 and 1892).

Taking everything into account, it will probably be well to construct standards either with oil insulation only, or to bake the coils in shellac before testing, instead of soaking in paraffin. Fig. 89 illustrates a form of an oil immersed standard which is in use in my laboratory, and through which a considerable current may be passed. The oil is stirred by means of a screw propeller.

Fig. 89.

Fig. 89 represents a standard resistance for making Clerk cell comparisons by the silver voltameter method. The framework on which the coils are wound consists of a base and top of slate. The pillars are of flint glass tube surrounding brass bolts, and cemented to the latter by raw shellac. Grooves are cut in the glass sleeves to hold the wires well apart. These grooves were cut by means of a file working with kerosene lubrication. A screw stirrer is provided, and the whole apparatus is immersed in kerosene in the glass box of a storage cell. The apparatus is aged to begin with by heating to a temperature a good deal higher than any temperature it is expected to reach in actual work. After this the rigidity of the frame is intended to prevent any further straining of the wire. The apparatus as figured is not intended to be cooled to 0° C, so that it is put in as large a box as possible to gain the advantage of having a large volume of liquid. The top and bottom slates measure seven inches by seven inches, and the distance between them is seven inches. The inner coil is wound on first, and the loop which constitutes the end of the winding is brought up to a suitable position for adjustment. The insulation of the heavy copper connectors is by means of ebonite.

§ 120. Platinum Iridium.

Platinum 90 per cent, iridium 10 per cent. This material was prepared in some quantity at the cost of the French Government, and distributed for test about 1886. Klemencic got some of it as representing Austria, and found it behaved very like the platinum silver alloy just discussed. The temperature coefficient is, however, higher than for platinum silver (0.00126 as against 0.00027). The mechanical properties of the alloy are, however, much better than those of the silver alloy; and in view of the experience with B. A. standards above quoted, it remains an open question whether, on the whole, it would not be the better material for standards, in spite of its higher price. Improvements in absolute measurements of resistance, however, may render primary standards superfluous.

§ 121. Manganin.

Discovered by Weston--at all events as to its application to resistance coils. A glance at the diagram will exhibit its unique properties, on account of which it has been adopted by the Physikalisch Technischen Reichsanstalt for resistance standards. The composition of the alloy is copper 84 per cent, manganese 12 per cent, nickel 4 per cent, and it is described as of a steel-gray colour. Unfortunately it is apt to oxidise in the air, or rather the manganese it contains does so, so that it wants a very perfect protection against the atmosphere.

Like German silver, manganin changes in resistance on winding, and coils made of it require to be artificially aged by heating to 150° for five hours before final adjustment. The annealing cannot be carried out in air, owing to the tendency to oxidation. The method adopted by St. Lindeck (at all events up to 1892) is to treat the coil with thick alcoholic shellac varnish till the insulation is thoroughly saturated, and then to bake the coil as described. The baking not only anneals the wire, but reduces the shellac to a hard and highly insulating mass.

Whether stresses of sufficient magnitude to produce serious mechanical effects can be set up by unequal expansion of wire and shellac during heating and cooling is not yet known, but so far as tested (and it must be presumed that the Reichsanstalt tests are thorough) no difficulty seems to have been met with. In course of time, however, probably the best shellac coating will crack, and then adieu to the permanency of the coil! This might, of course, be obviated by keeping the coil in kerosene, which has no action on shellac, but which decomposes somewhat itself.

The method of treatment above described suffices to render coils of manganin constant for at least a year (in 1892 the tests had only been made for this time) within a few thousands per cent. Manganin can be obtained in sheets, and from this material standards of 10-2, 10-3, and 10-4 ohms are made by soldering strips between stout copper bars, and these are adjusted by gradually increasing their resistance by boring small holes through them. The solder employed is said to be "silver."

Mr. Griffiths (Phil. Trans. vol. clxxxiv. [1893], A, p. 390) has had some experience with manganin carrying comparatively heavy currents, under which circumstances its resistance when immersed in water was found to rise in spite of the varnish which coated it. Other experiments in which the manganin wire was immersed in paraffin oil did not exhibit this effect, though stronger currents were passed.

On the whole, manganin appears to be the best material for coil boxes and "secondary" resistance standards. Whether it is fit to rank with the platinum alloys as regards permanency must be treated as an open question.

§ 122. Other Alloys.

The following tables, taken from the work of Feussner and St. Lindeck, Zeitschrift fuer Instrumenten Kunde, 1889, vol. ix. p. 233, together with the following notes, will suffice.

§ 123. Nickelin.

This is only German silver with a little less zinc, a little more nickel, and traces of cobalt and manganese. It behaves like German silver, but is an improvement on the latter in that all the faults of German silver appear upon a reduced scale in nickelin.

§ 124. Patent Nickel.

Practically a copper nickel alloy, used to some extent by Siemens and Halske. It stands pretty well in the same relation to nickelin as the latter does to German silver. After annealing as for manganin it can be made into serviceable standards which do not change more than a few thousandths per cent. I have not come across a statement of its thermo-voltage against copper.

§ 125. Constantin.

Another nickel copper alloy containing 50 per cent of each constituent. It appears to be a serviceable substance, having a temperature coefficient of 0.003 per cent per degree only, but an exceedingly high thermo-voltage, viz. 40 micro-volts per degree against copper.

1 2 3 4 5 6 7 8
German Nickelin made Rheo- Patent Nickel Manga- Nickel
Silver by Obermaier tane nese Manga-
Dia- Dia- Dia- Dia- Copper nese
meter meter meter meter Copper
1.0mm 0.1mm 0.6mm 1.0mm

Copper 60.16 61.63 54.57 53.28 74.41 74.71 70 73

Zinc 25.37 19.67 20.44 16.89 0.23 0.52 ... ...

Tin ... ... ... ... trace ... ...

Nickel 14.03 18.46 24.48 25.31 25.10 24.14 ... 3

Iron 0.30 0.24 0.64 4.46 0.42 0.70 ... ...

Cobalt trace 0.19 ... ... trace trace ... ...

Mang- trace 0.18 0.27 0.37 0.13 0.17 30 24 anese.

99.86 100.37 100.40 100.31 100.24 100.24 ... ...

Specific resistance 30.0 33.2 44.8 52.5 34.2 32.8 100.6 47.7

Temperature coefficient 0.00036 0.00030 0.00033 0.00041 0.00019 0.00021 0.00004 0.00003

The specific resistance is in microhms, i.e. 10-6 ohms per cubic centimetre, and the temperature coefficient in degrees centigrade.

126. Nickel Manganese Copper.

I can find no other reference with regard to this alloy mentioned by Lindeck. Nicholls, however (Silliman's Journal [3], 39, 171, 1890), gives some particulars of alloys of copper and ferromanganese. The following table is taken from Wiedemann's Beiblatter (abstract of Nicholl's paper, 1890, p. 811). All these alloys appear to require annealing at a red heat before their resistances are anything like constant.

Let x be percentage of copper, then 100--x is percentage of "ferromanganese."

Values of x. 100 99.26 91 .88 86.98 80.4 70.65

Specific resistance with respect to copper (? pure) 1 1.19 11.28 20.4 27.5 45.1

Temperature coefficient per degree x 10^6(hard) 3202 2167 138 16 22 -24

Ditto (soft) ... ... 184 80 66 21

If nickel is added, alloys of much the same character are obtained, some with negative temperature coefficients--for instance, one containing 52.51 per cent copper, 31.27 per cent ferromanganese, and 16.22 nickel.

A detailed account of several alloys will be found in a paper by Griffiths (Phil. Trans. 1894, p. 390), but as the constants were determined to a higher order of accuracy than the composition of the material--or, at all events, to a higher degree of accuracy than that to which the materials can be reproduced--there is no advantage in quoting them here.